It is now well established that, in mammals, sulfate conjugation const
itutes an important reaction in the transformation of xenobiotics and
in the modulation of the biological activity of steroid hormones and n
eurotransmitter (1, 2). The presence of a sulfate group on some molecu
les can also be a prerequisite for their biological function. For exam
ple, it is well known that the sulfate groups are directly involved in
the molecular interaction between heparin and antithrombin III (3). I
n plants, sulfation also seems to play an important role in the interm
olecular recognition and signaling processes, as indicated by the requ
irement of a sulfate moiety for the biological activity of gallic acid
glucoside sulfate in the seismonastic and gravitropic movements of pl
ants (4), and of Nod RM1 in the cortical cell division during early no
dule initiation in Rhizobium meliloti-alfalfa interaction (5). In addi
tion, recent studies indicate that flavonoid conjugates, including the
sulfate esters, may play a role in the regulation of plant growth by
strongly binding the naphthylphthalamic acid receptor, thus blocking t
he quercetin-stimulated accumulation of the auxin phytohormone (6). Al
though several sulfated metabolites are known to accumulate in a varie
ty of plant species (7), the study of enzymes that catalyze the sulfat
ion reaction in plants lagged considerably compared to those conducted
with their mammalian homologs. This apparent lack of interest may hav
e been because the function of plant-sulfated metabolites is difficult
to predict, since their accumulation is often restricted to a limited
number of species. Despite this limitation, several plant sulfotransf
erases (STs) have been characterized at the biochemical level, and the
cDNA clones encoding six plant STs have been isolated. Based on seque
nce homology, the plant ST coding sequences are grouped under the SULT
3 family, also known as the flavonol ST family. This review summarizes
our current knowledge of the plant STs and focuses on the functional
significance of the sulfate conjugation in plant growth, development,
and adaptation to stress.